Presently, a specification of the LTE-Advanced system, which is an evolved version of the Long Term Evolution (LTE) system, is studied by the 3rd Generation Partnership Project (3GPP). The LTE-Advanced system has, for example, a configuration as described below. Specifically, the LTE-Advanced system includes a base station or a base station device (hereinafter collectively referred to as “base station”) called as evolved Node B (eNB), and a communication terminal, terminal, subscriber unit, or terminal device (hereinafter collectively referred to as “communication terminal”) called as User Equipment (UE). The base station is a transmission device, a transmitter, or a transmission station that transmits a downlink signal to the communication terminal, and also is a reception device, a receiver, or a reception station that receives an uplink signal from the communication terminal. Likewise, the communication terminal is a reception device, a receiver, or a reception station that receives the downlink signal from the base station, and also is a transmission device, a transmitter, or a transmission station that transmits the uplink signal to the base station. The LTE-Advanced system includes a Mobility Management Entity (MME) which is a controller constituting the core network, and a Serving Gate Way (S-GW) which is a server for transmitting data such as user data. Further, the LTE-Advanced system includes a S1 which is an interface between the MME/S-GW and the eNB, and an X2 which is an interface between eNBs. The S1 and the X2 are interfaces using the GPRS Tunneling Protocol (GTP) based on the Transmission Control Protocol/Internet Protocol (TCP/IP).
Then, the base station forms a cell defined with the frequency and the service area or the communication area, communicates with a communication terminal accommodated in the cell, and communicates with another base station, thereby enabling communication terminals accommodated in the same cell or different cells to communicate with each other.
The LTE system is capable of setting the uplink/downlink bandwidths or system bandwidth to 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz. Each of the bands thus set is defined as Component Carrier (hereinafter sometimes referred to as “CC”). The reason why multiple bandwidths may be set as above is that the LTE system is based on the premise that bandwidths allocated to conventional Global Systems for Mobile communications (GSM) (registered trademark) and Wideband Code Division Multiple Access (W-CDMA) are used in the LTE system.
Here, the 3GPP defines the “cell” as “a service area formed using one frequency”, that is, “a service area covered by one frequency”. One base station has one band only. Further, one cell is formed for one CC, and the cell and the CC or the band are in a one-to-one correspondence. Thus, in the 3GPP, “base station”, “cell”, “band”, and “CC” may be handled as being synonymous with each other. Description below is given based on the above premise. In practice, one base station may use a plurality of bands and include a plurality of sectors (which correspond to cells in the 3GPP). In this case, the technique disclosed herein may be applied in the same manner as described below, unless otherwise specified.
The cell is a band which is segmented from a band allocated to one communication system (for example, W-CDMA system and LTE system) based on a bandwidth (system bandwidth) constituting the system. Thus, user multiplexing or multiple access is available in each band. Further, user multiplexing is possible by allocating a radio resource of the data channel using the band to one or more communication terminals by scheduling. In other words, the cell may constitute one communication system and is different from a block, a resource block, a group, or a cluster into which a plurality of sub-carriers are aggregated as the radio resource allocation unit for user multiplexing in the Orthogonal Frequency-Division Multiple Access (OFDMA).
Here, since the LTE system is desired to achieve a faster transmission than the conventional GSM systems and the W-CDMA systems, the bandwidth is desired to be wider than those of these communication systems. Meanwhile, a band used in the wireless communication system is generally different depending on circumstances of individual countries. Further, in Europe where two or more countries are accessible each other by land and share their border each other, the frequency band in use is adjusted between countries in consideration of the interference. As a result, the number of bandwidths available for the wireless communication system in individual countries is reduced, and the bandwidth is chopped. In view of the problem, a technique of providing a wide band by aggregating narrowed and chopped bands is introduced to achieve a wide band in the LTE system.
As a technique for achieving the wide band, a technique called as Carrier Aggregation (hereinafter alternatively referred to as “CA”) is studied for the LTE-Advanced system. The CA is a technique to communicate by simultaneously using a plurality of frequency bands. Specifically, the CA is a technique to communicate between at least one transmission device and at least one reception device simultaneously using a plurality of frequency bands, or a technique to communicate between one transmission device and at least one reception device simultaneously using a plurality of frequency bands. If these are satisfied, the name of the technique for achieving the wide band is not limited to the CA. When data is transmitted using a certain frequency, the frequency used for transmission of data generally has a bandwidth. Therefore, “frequency band” and “frequency” may be synonymous with each other hereinafter.
When implementing the CA, a main cell is established first. The main cell in the CA is called as primary cell. The primary cell is sometimes called as first cell, first band, main band, or main cell. Hereinafter, the primary cell may be referred to as “PCell”.
Then, in the CA, the cell is added or aggregated into the PCell. The cell added to the PCell is called as secondary cell. The secondary cell is sometimes called as second band, extended band, or subband. Hereinafter, the secondary cell may be referred to as “SCell”.
In the LTE Release 10-12, seven SCells may be established at the maximum in the CA. Specifically, the CA may be achieved by using eight CCs including the PCell at the maximum. Presently, achieving of establishing 32 CCs at the maximum is studied. In other words, the CA is a technique to aggregate the PCell and at least one SCell. The CA is classified depending on whether a frequency of the PCell and a frequency of the SCell is continuous (contiguous/non-contiguous) and whether the frequencies are included in the same frequency band (intra frequency band/inter frequency band). Further, the CA is classified depending on whether control information for data communication using the SCell is transmitted by the SCell (straight scheduling) or by the PCell or another SCell (cross carrier scheduling). Here, the Physical Downlink Shared Channel (PDSCH) being a downlink shared channel is used for data communication using the SCell. On the other hand, the Physical Downlink Control Channel (PDCCH) being a downlink control channel is used for transmission of the control information for data communication using the SCell.
For example, to introduce the CA into a communication system, a cell configuration including the PCell which is a cell having a wider area and the the SCell which is a cell having an area narrower than the PCell is studied. In this cell configuration, at least a portion of the area in the SCell overlaps the PCell. A cell having a wider area may be called as macro cell. A cell having a narrower area may be called as micro cell, pico cell, femtocell, or small cell.
Frequency band used for the cellular system is determined by a decree in consideration of circumstances of individual countries based on the international frequency allocation. The cellular system includes, for example, the Wideband Code Division Multiple Access (W-CDMA) system, the LTE system, the LTE-Advanced system, and the Worldwide interoperability for Microwave Access (WiMAX) (registered trademark) system.
Further, the frequency band used for the cellular system is allocated to communication providers by using, for example, auctions among communication providers. Specifically, a license is granted by designating a used frequency band for each of communication providers, and thereby communication providers are permitted to use the designated frequency band. The frequency band thus permitted to use by the license is called as “licensed band” or “frequency for which a license has to be possessed”. In other words, the licensed band is a frequency band of the license system. The licensed band is a frequency band that a specific communication provider permitted to use the licensed band is allowed to use exclusively.
Meanwhile, there is a communication system that is allowed to communicate without the license by communicating with a transmission power equal to or lower than the maximum transmission power specified by the decree. Such communication system is called as specific small power system. Frequency bands such as the Industry Science Medical (ISM) band and the 5 GHz band using a transmission power equal to or lower than the transmission power specified by the decree may be used freely without the license. Such frequency bands that may be used without the license are called “unlicensed band” or “frequency for which a license does not have to be possessed”. In other words, the unlicensed band is a frequency band of the non-license system. Thus, since the unlicensed band is a frequency band that may be used freely without the license, exclusive use of the unlicensed band by only specific communication providers is not permitted. In other words, since the unlicensed band may be used freely by all communication providers, exclusive use of the unlicensed band by only specific communication providers is not permitted. Thus, premise of the unlicensed band is temporary use thereof. The communication system using the unlicensed band includes, for example, the Wireless Fidelity (Wi-Fi) system using the ISM band (IEEE 802.11a).
In recent years, licensed bands used for communication are added one by one to cope with increasing communication traffic. For example, a new 3.5 GHz band is added to an existing 1.7 GHz band. However, since frequency resource is finite, increasing licensed band used for communication depletes the number of remaining frequencies. Thus, it is difficult to cope with the increasing communication traffic just by increasing the licensed band used for communication.
To solve the problem, use of the unlicensed band used in the Wi-Fi system in the LTE system and the LTE-Advanced system (cellular system) is studied. In other words, use of the unlicensed band in the LTE system and the LTE-Advanced system in addition to the licensed band is studied.
For example, it is studied that in implementing the CA, the licensed band in the LTE system is used as the PCell and the unlicensed band in the Wi-Fi system is used as the SCell (first study). According to the first study, the CA is implemented by simultaneously using multiple Radio Access Technologies (RATs) different from each other: the LTE and the Wi-Fi. Communication simultaneously using multiple RATs different from each other may be called as system aggregation. In the 3GPP, the first study is under way as a dual connectivity using the LTE and the Wi-Fi.
For example, it is studied that in implementing the CA, the licensed band in the LTE system is used as the PCell and an unlicensed band to which the LTE-Advance system is applied is used as the SCell (second study). In the 3GPP, the second study is under way as the Licensed-Assisted Accessing in LTE (LAA).
Further, control to use the unlicensed band as the SCell by the licensed band used as the PCell may be called as licensed assisted.
Examples of the related art include Japanese Laid-open Patent Publication No. 2003-018642, No. 2008-103959, No. 2009-207108, and No. 2013-042258, Japanese National Publication of International Patent Application No. 2013-545365, No. 2014-529276, and No. 2015-505436, Japanese Patent No. 4515460, International Publication Pamphlet No. WO 2008/090603, No. WO 2009/020017, and No. WO 2010/073468, Non Patent Literature 1: TS36.211V8.9.0 “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, and Non Patent Literature 2: TS23.003V8.16.0 “3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Numbering, addressing and identification (Release 8).”